Traditionally, in the known headrests of the above-referenced type the support structure comprises a pair of parallel rods projecting from the base of the yielding body to be connected to the structure of a seat backrest. Superiorly the two support rods are connected to a load-bearing framework incorporated within the headrest yielding body, and adjustment means are normally provided for the height adjustment of the yielding body relative to the seat backrest. These adjustment means are usually arranged according to two alternative systems: in a first system the support rods are rigidly fixed to the load-bearing framework of the yielding body, and are lowerly connected to the backrest structure in a vertically slidable way, with the aid of resilient positioning members releasably co-operating with corresponding notches formed in the lower portions of the support rods. In the second system the two support rods are rigidly secured to the backrest structure, and are slidably coupled superiorly to the load-bearing framework of the yielding body. Even in this case resilient positioning members are provided, releasably co-operating with notches formed in the upper portions of the support rods to retain the headrest yielding body in the selected position.
These traditional headrests are affected by several drawbacks. Firstly, the slidable connection between the two support rods and the backrest structure or, respectively, the load-bearing framework incorporated within the headrest yielding body may be subjected in use to slacks originating vibrations and noise. Moreover this slidable connection between the support rods and the yielding body, or respectively, the seat backrest may involve undesired upward or downward displacement of the headrest and also, in case of abrupt crash of the vehicle on which the headrest in installed, accidental withdrawal of the support rods and separation of the yielding body from the seat backrest. In such event the headrest function would fail, with adverse consequences for the user.